Chemical deposition of PbS from an acidic bath

Thin Sohd Ftlms, 245 (1994) 7-9

7

Letter

Chemical deposition of PbS from an acidic bath K. M. Gadave, S. A. Jodgudri and C. D. Lokhande Thin Fdm Physws Laboratory, Department of Physws, Shwajt Unwerstty, Kolhapur-416 004 (Indm)

(Received January 17, 1993, accepted February 23, 1994)

Abstract Preparation of lead sulphide films from an aqueous acidic medium has been carried out using thiosulphate as a sulphurion releasing source. The films are polycrystalhne and very adherent to the substrate Optical absorpuon and IR studies were carried out, and the electrical resistivity is of the order of 105-106 ~ cm.

1. Introduction PbS, with its direct bandgap of 0.4 eV and its absorption coefficient continuously increasing from the inrared ( 150 cm- ~ at 3 lam) through the visible region, has been used in infrared detectors since the mid 1940s [1, 2]. It was for this application that the chemical deposition technique for PbS thin films, known since 1910 [3], was initially developed in the late 1940s [4]. The typical deposition process involved the immersion of glass substrates in alkaline lead-thiourea solutions [ 1, 5]. Later, in solar charge research, PbS thin films were investigated for photothermal conversion applications, either independently on metallic substrates [6, 7] or in multilayer stacks of PbS-CdS-PbS [8], (PbS)~_x(CdS)x composites [9] and in PbS and non-reflective layer combinations [10]. Solar control applications of PbS thin films 0.05-0.15 tam thick deposited them on glass substrates In single layers [11-13]. Reddy et al. have prepared PbS on tin-oxide-coated glass and ZnO substrates. Acharya and Bose [14] have reported the chemical deposition of PbS using lead acetate, thlourea and ammonia solutions. Sharma et al. [15] deposited epitaxial films of Pbt_xHgxS (x = 0 0.33) on Ge and Si single crystal substrates from an alkaline medium using lead acetate, mercuric chloride and thiourea in the temperature range 10-90 °C. Electron microscopy and electron diffraction studies show the predominance of the (11 I) and (112) orientations on (11 l)Ge and ( 111)Si substrates, respectively. Nair et

al. [16] have reported the chemical deposition of PbS

thin films and its applications in decorative coatings and imaging techniques. Sharma et al. [17] prepared copper-doped PbS films and studied the dependence of structure, electrical conductivity and carrier mobility with doping. It has been observed that the copper doping affects the grain size and structure of the PbS films indicating formation of copper lead alloy. In this paper, we report on PbS film deposition from acidic medium, using lead acetate and sodium thiosulphate at a bath temperature of 80 °C. Their electrical and optical properties are also studied.

2. Experimental procedure For the preparation of PbS films using the chemical bath deposition method in acidic media, equimolar (0.1 M) solutions of Pb(CH3COO)2 and Na2S203 were added in a beaker. The solution was stirred continuously using a magnetic stirrer. The pH of the solution was between 5 and 6. Ultrasonically cleaned glass substrates were dipped in the solution. The bath temperature was maintained at 80 °C. After one hour, the films were taken out of the bath, washed with doubly distilled water, dried and preserved. The colour of the film was greyish-black. The X-ray diffraction (XRD) pattern was taken with a Philips PW-1710 machine. Microstructural and optical absorption studies of the films were carried out with a scanning electron microscope (Cambridge Stereoscan -250 MK-3 unit) and a U V - V I S - N I R spectrophotometer respectively. The electrical resistivity was studied by the two probe method making silver contacts to the films. IR spectra were recorded with a Perkin-Elmer spectrophotometer.

3. Results and discussion The reaction mechanism for deposition of the metal sulphide films from an acidic bath using Na2S203 as a sulphur source has previously been reported [18-20]. The reaction mechanism for PbS deposition is proposed as follows. NAzS203 is a reducing agent by virtue of the half cell reaction 2S2032

--,

S4062 -~ 2e-

(1) Elsevier Sequoia

SSDI 0040-6090(94)09455-G

In the acidic medmm, dissociation of Na2S20~ takes place $20~- + H " ~ H S O ; + S

-----7

i

(2)

The electrons released (reactton (1)) react with sulphur from reaction (2) S + 2e- ~ S-=

600

z.00 >-

(3) w

and PbS is formed as Pb ~-2 + S - 2 ~ PbS

Z

- 20C

(4)

Figures l(a) and l(b) show scanning electron microscopy mzcrographs of the PbS film on a glass substrate at two different magnifications. Figure l(a) shows good coverage of the substrate and overgrowth of PbS particles on the film surface, while Fig. l(b) shows individual clusters of PbS in great detail. The X-ray diffraction pattern of the PbS films is shown in Fig. 2. The well-defined (200) peak is observed m the X R D pattern. The films are polycrystalline. Table 1 shows the comparison of the observed d values with the ASTM data. Fzgure 3 shows the optical absorption and transmissxon of the PbS film studied in the wavelength range

0

~'o

~o

~o

60 so 2 o" --~

70

80

90

1O0

Fzg 2 X-ray &ffractxon pattern of PbS film 50

I00

BO ta.I

•,el

W

so~

Al:6orption

t,r taJ

40 ~ m

Z0

1,0 ~' I

Trar~'n~on

AvE

F~g 3. Variation of optical absorptzon and percentage transmtssmn with wavelength for the PbS film.

!

ta)

F ....

~V6o0 '

3d00 '

2o'0o '

Fig. 4 The IR spectrum of PbS powder collected after film deposltmn

TABLE I. Comparison of observed d values for PbS thin films with standard ASTM data

(b) F~g I SEM mlcrographs of PbS films wtth magmficatlons (a) 3000× (b) 500x

Observed d values (/~)

Standard d values (,~,)

Planes

1/lo

3.3349 3 0155 2 0878 1 7859 1.4827 13184

3 3984 2 9987 2 0924 1 7822 1 4805 1 3248

111 200 220 311 400 420

23 41 100 8t 18 62 l0 15 6 05 2 59

Letter

4 0 0 - 2 0 0 0 nm. Nearly 100% absorption is observed in the studied wavelength range. The IR spectrum of PbS powder collected from the bath and made acid free, studied in the 2 0 0 - 4 0 0 0 cmrange, is shown m Fig. 4. This matches well with the standard IR spectrum of PbS [21]. The peaks at 6 4 0 610 cm -t and 2300-2340 cm -~ are due to atmospheric CO2 [21]. Smaller but sharper peaks between 1600 and 1800 cm- 1are due to atmospheric water vapour and CO2.

Conclusion From the above results it is concluded that chemical deposition of PbS films from aqueous acidic medium can be carried out at 80 °C. The films are very adherent to the substrates and are polycrystalline. The electrical resistivity is of the order of 105-106 ~ cm.

Acknowledgment The authors are grateful to U G C New Delhi for financial support under project No. F- 10-96/90 (RBB-II).

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